In wireless communication networks, such as cellular networks like those complying with the UMTS (Universal Mobile Telecommunication Standard), the transmission channel on which data streams are transmitted suffers from attenuation due to destructive addition of multipaths in the propagation media (i.e., the air) and to interference from other users. Moreover, the transmitted data streams have to travel across a potentially harsh environment, experiencing scattering, reflection, refraction and so on, and they may be corrupted by thermal noise.
In order to overcome these problems, a known wireless transmission technique provides for encoding the data to be transmitted exploiting a so-called Space-Time Block Code (STBC). By using an STBC, the data to be transmitted is encoded in blocks; then, multiple copies of the data stream (formed by copies of said blocks) are transmitted by means of a plurality of antennas. In this way, by exploiting the various received versions of the data stream it is possible to greatly improve the reliability of the transmission. Using multiple antennas at the receiver side further improves the performances.
The transmission of different copies of the data stream is also known as “transmission/reception diversity scheme”. This technique has been studied extensively because it is considered relatively simple to have multiple transmitting antennas. The “diversity gain” provides an indication of the reliability of this method. Briefly, the diversity gain represents the increase in the signal-to-interference ratio due to the adopted diversity scheme, or how much the transmission power can be reduced when the diversity scheme is introduced, without a performance loss.
In order to exploit this technique in a mobile wireless network (e.g., a network that adopts a Code Division Multiple Access—CDMA—modulation scheme), the mobile terminals (e.g., mobile phones) have to be equipped with dedicated hardware capable of performing proper combining and/or selecting operations on the various received versions of the data stream, in order to improve the quality of the received data. This dedicated hardware is expensive, both in terms of cost and power consumption.
A simple transmission diversity scheme proposed by Alamouti provides for the use of two antennas on the transmitter side. The diversity gain that can be obtained using the Alamouti scheme is equal to that achievable by applying a Maximal Ratio Receiver Combining (MRRC) with two antennas on the receiver side.
The Alamouti scheme is less complex than the known diversity schemes that adopt space-time trellis coding. However, this greater simplicity implies a loss of performance, that in some cases might not be acceptable. In order to improve the performance of the transmission, a similar transmission diversity scheme can be exploited using more than two transmitting antennas. For this purpose, in order to have the highest possible diversity gain (and, consequently, the highest performances) a mobile terminal on the receiver side needs to be equipped with hardware resources adapted to receive and decode data streams transmitted exploiting different transmission diversity schemes. Indeed, a mobile terminal specifically designed for receiving and decoding data streams transmitted and coded with, e.g., only the Alamouti scheme, is not capable of guaranteeing a high diversity gain in case the transmission is performed by a different number (e.g., four) of antennas on the transmitter side.